1. Field of the Invention
The apparatus and method of the present invention relates to wireline operations in the recovery of oil and gas. More particularly, the present invention relates to a method and apparatus for controlling well pressure while undergoing wireline operations on subsea blowout preventers on the subsea floor.
2. General Background of the Invention
In conducting wireline operations, many types of tools are positioned on the lower end of a wireline, which is a steel cable or the like, lowered into the well bore in order to undertake certain tests downhole. Because in the past there have been difficulties in undertaking wireline work with the potential hazard of blowouts in the well, there has been developed and patented by Harper Boyd, a side entry sub assembly which is patented under U.S. Pat. No. 4,681,162 and reissued under U.S. RE 33,150. This patented device, which is placed below the top drive on the rig floor, includes a side entry portion which enables the wireline to extend through the side entry passage and into the main passage and downward into the drill string. Although the use of the side entry sub is common for drilling, the system has never been applied safely on subsea blowout preventers which are located in deep water, since controlling the well pressure at deep depths is very difficult. In deep waters of the Gulf of Mexico, there would be provided a floating subsea riser which would extend from the rig floor, on the Gulf surface, to the blowout preventers on the floor of the Gulf, sometimes some 4,000 to 10,000 feet in distance. It would be quite impractical to run a high-pressure line from the rig floor to the BOP stack on the Gulf floor to tie into the annular preventer so that one could pressure test the wireline. The BOP's need to be pressure tested, but the riser cannot take high pressure tests above the blowout presenters, or it would rupture and expel hydrocarbons into the Gulf waters. So, there is a need to be able to conduct subsea wireline operations in deep waters under pressure so that in the event a well would “come in” during the operations, the blowout would not reach the riser to the rig floor to avoid rupture of the floating subsea riser and a major catastrophe.